1. Field of the Invention
The present invention relates to an automatic focussing adjusting device the, in particular, to an automatic focussing adjusting device which controls and integration time necessary to integrate the amount of light received by an CCD line sensor which measures light for detection of a focal position.
2. Description of the Related Art
In general, in an automatic focussing adjusting device for use in an optical instrument such as a camera or the like, a control device such as a microcomputer or the like analyzes the order of focussing in accordance with the signal that is output from a CCD line sensor for detecting a focus position according to the amount of light received from an object, and adjusts the position of a focus lens of a taking lens system, so that an accurate focussing can be provided.
In the CCD line sensor of the above-mentioned type, for example, 2 kinds of sensors, each comprising a plurality of light receiving elements which are arranged linearly are arranged at positions in which the respective optical paths are different from each other, and part of the light taken out of the light entering through a taking lens is focussed on the light receiving surfaces of the respective receiving elements of the two sensors. The respective sensors output electric signals having wave forms that correspond to the intensity of illumination of the object image focussed, and the quantity of de-focussing of the focal position of the focus lens is operated in accordance with the correlation between the wave forms of the two electric signals. Usually, the output level of the CCD line sensor is determined by controlling a gate time; for example, when the output level exceeds the saturation level of the CCD line sensor, the light receiving amount is controlled by shortening the next gate time over the current gate time. This is equivalent to controlling the next CCD integration time t.sub.N of the amount of the light received by the CCD line sensor, and the integration time t.sub.N is usually determined by an equation (1): ##EQU1## where t.sub.T represents the current integration time of the CCD line sensor, D.sub.R represents a reference value in the range of stable operation the CCD line sensor, that is, it stands for a reference value set within a predetermined operation range which is higher than the lowest output level and lower than the saturation level, and D.sub.A stands for an average value of the output signals of the CCD line sensors which are the integration values thereof. As mentioned above, the next integration time t.sub.N of the CCD line sensor is a value which can be obtained by increasing or decreasing the current integration time t.sub.T according to D.sub.R /D.sub.A so that the average value D.sub.A always converges to the reference value D.sub.R. In this case, the average value D.sub.A is calculated intact according to the brightness of the light of an object which varies floatingly, and thus even when the brightness changes suddenly, the average value D.sub.A can be obtained accurately.
However, in the above-mentioned method of deciding the integration time t.sub.N of the CCD line sensor for the light measurement, due to the fact that the average value D.sub.A is calculated intact according to the always varying brightnesses of the object light, when the brightness varies remarkably, the integration time t.sub.N is caused to vary to a great extent. Accordingly, with the result that the average value D.sub.A goes out of the stable operation region of the CCD line sensor and becomes a value in the unstable operation region thereof, good CCD data can not be obtained.
When the average value D.sub.A is a value present in the stable operation region of the CCD line sensor but the brightness of the field light continues to vary slightly, the integration time t.sub.N for the amount of light received by the CCD line sensor is caused to vary greatly in response consequently, the average value D.sub.A is caused to vary with respect to the reference value D.sub.R, so that a delicate error in distance measurements occurs which provides an unstable condition.